1. Field of the Invention
The present invention relates to a method of applying a metallic paint using a rotary atomizing electrostatic painting machine and, more particularly, to a method of applying a metallic paint by reciprocating a painting machine at right angles to the direction in which an object to be painted is conveyed at a constant speed.
2. Description of the Prior Art
Since rotary atomizing electrostatic paint spraying can apply paint at a higher transfer efficiency than air atomizing electrostatic spraying and airless atomizing electrostatic spraying, it has often been used to paint automotive bodies and the like in recent years.
FIG. 4 shows the conventional rotary atomizing electrostatic spray equipment. The body of this equipment is indicated by numeral 1. A rotor 3 is supported by an air bearing 2 inside the body 1 and extends out through the body 1. An atomizing head 4 is firmly fixed to the front end of the rotor 3 which is located outside the body 1. A head member 5 is mounted to the front end of the body 1 and provided with a multiplicity of blowoff holes 6 for ejecting shaping air toward the outer fringe of the atomizing head 4. A paint supply tube 7 extends along the axis of the body in such a way that it is not in contact with the rotor 3.
In the operation of the conventional rotary atomizing electrostatic spray equipment constructed as described above, the atomizing head 4 is spun at a high speed by a drive means (not shown) and, at the same time, a high voltage is applied to the head 4. Under this condition, a paint is supplied through the paint supply tube 7. Then, the paint flows out to the front surface of the head 4 from a hole 4a formed in the head 4, runs across the front surface, and shifts to the outer periphery. Then, the paint is atomized by centrifugal force. Concurrently, the atomized paint is electrically charged and travels toward the object to be painted. At this time, the paint is applied in the desired pattern by the shaping air ejected from the blowoff holes 6.
It is known that where a metallic paint including fragments of aluminum or mica is applied using the above-described rotary atomizing electrostatic spray equipment, the appearance of the finished object is much darker than the case in which it is painted by the use of an air atomizing spray machine. This phenomenon is understood in the manner described now. Since the particles of the paint are applied to the target surface mainly by electrostatic force, the speed of the paint particles colliding against the sprayed surface is lesser than in the air atomizing electrostatic spraying. Therefore, the fragments of aluminum or mica do not easily orient themselves parallel to the sprayed surface. Japanese Patent Laid-Open No. 315361/1989 discloses a painting method which solves the above problem. In particular, the pressure created by shaping air is increased to increase the speeds at which the paint particles collide against the object to be painted, for enhancing the lightness of the surface sprayed with the metallic paint. This technique yields considerably satisfactory results.
Another painting method has been used in which a painting machine is reciprocated perpendicularly to the direction of conveyance of a painted object conveyed at a constant speed. This method is disclosed, for example, in Japanese Patent Laid-Open Nos. 165064/1982 and 255170/1985. In this reciprocating painting method, the reciprocating painting machine draws a sinusoidal orbit on the sprayed surface of the painted object, as shown in FIG. 5. This method permits an object having a wide painted area such as an automotive body to be painted efficiently.
Accordingly, where an object having a wide coated area is painted with a metallic paint through the use of a rotary atomizing electrostatic spraying machine, this machine, of course, must be reciprocated.
However, in the painting method disclosed in the above cited Japanese Patent Laid-Open No. 315361/1989, the pressure created by the shaping air is increased. As a result, a negative pressure is produced ahead of the atomizing head. The particles of the paint are attracted into the region where this negative pressure is developed. This reduces the width of the pattern in which the paint is applied. This reduction in the width of the depositional pattern means an improvement of the transfer efficiency of the paint. If the aforementioned painting method is directly applied to the reciprocating painting to spray a metallic paint, and if the speed of the conveyor varies at all, then the boundary between the adjacent orbits drawn by the spraying machine becomes conspicuous. That is, so-called nonuniform paint spraying tends to occur. It substantially follows that the utilization of this method is urged to be abandoned. Japanese Utility Model Laid-Open No. 13557/1987 discloses another rotary atomizing electrostatic spraying machine in which shaping air is tilted outward about the axis of the atomizing head to widen the spraying pattern. This machine also inevitably produces a negative pressure ahead of the atomizing head. Therefore, this machine is unsuccessful in essentially solving the above-described problem.